GUANTANAMO BAY—As the historic trial of the alleged architects of the Sept. 11, 2001 attacks grappled Tuesday with questions of secrecy, constitutional law and the defendants’ court attire, Washington’s Court of Appeals attacked the legitimacy of Guantanamo’s war crimes court in overturning the conviction of Osama bin Laden’s former driver.

Yemeni Salim Ahmed Hamdan was found guilty in 2008 by a military jury of a single charge of “providing material support to terrorism.” He was sentenced to 5 1/2 years but released to Yemen shortly after his conviction because he had already spent six years in custody here.

The Toronto Star was the only news outlet to interview Hamdan after his conviction, spending three days with him at his home in 2009.

He said at the time that he spoke reluctantly and only to bring attention to the Yemeni prisoners still detained here. “Hamdan this, Hamdan that, Hamdan all the time,” he said in English with a wave of his hand, weary of the attention already given to his case.

Tuesday’s ruling by the U.S. Circuit Court of Appeals for the District of Columbia overturned Hamdan’s conviction on the basis that the charge was not considered a war crime under international law at the time he chauffeured the Al Qaeda leader. Hamdan was charged under the 2006 Military Commissions Act and the three-judge panel ruled that he could not be retroactively prosecuted.

The ruling could also affect Australian David Hicks, the kangaroo skinner-turned-jihadist who pleaded guilty to the same charge in 2007 in return for a nine-month prison sentence to be served in Australia.

Canadian Omar Khadr also pleaded guilty to “providing material support for terrorism” as part of his 2010 deal. But unlike the others, Khadr’s record includes four additional Guantanamo convictions including “murder in violation of the laws of war.”

Critics of Guantanamo said the D.C. court’s ruling undermines the military commission process, tarnishing this week’s pretrial hearings against self-professed Sept. 11 mastermind Khalid Sheikh Mohammed and his four co-accused.

“This decision strikes the biggest blow yet against the legitimacy of the Guantanamo military commissions, which have for years now been trying people for a supposed war crime that, in fact, is not a war crime at all,” the American Civil Liberties Union’s Zachary Katznelson said Tuesday.

He said the ruling also calls into question the military commission charge of “conspiracy” faced by the five Sept. 11 suspects, because it is also not a traditional war crime. “By continuing with a conspiracy charge in this case, the government is virtually ensuring further appeals and casts even further doubt on the legality of these proceedings,” he said.

Mohammed, his nephew Ali Abdul Aziz Ali, and Saudi Mustafa al Hawsawi did not come to court Tuesday, voluntarily opting out of the proceedings, which the judge confirmed on Monday was their right. Only Yemeni prisoners Walid bin Attash and Ramzi bin al Shibh appeared in court, spending most of the day reading from thick legal binders.

But if Mohammed does attend future hearings, he may be sporting a camouflage vest.

Debate over the defendants’ clothing in court dominated most of Tuesday morning’s proceedings.

Army Judge Col. James Pohl ruled that the accused could wear camouflage as they requested — but which had been forbidden by the prison’s commander — as long as it was not part of a U.S. military uniform.

Mohammed’s lawyers had argued that the 47-year-old had worn such a vest when he was a mujahedeen fighter in Bosnia and Afghanistan — an issue that may take on greater significance when debating the Geneva Convention protections concerning armed groups.